1. Field of the Invention
The present invention relates generally to a method for making glass, and particularly to a method for making inclusion free homogeneous glass.
2. Technical Background
Integrated circuits (ICs) are fabricated using microlithographic systems. The goal of IC manufacturers is to produce integrated circuits having linewidths as small as possible. Most of the microlithographic systems currently in use employ transmission optics. The typical system includes an illumination source coupled to an illumination optics assembly which provides a photomask with illumination light. The illumination optics expand and collimate the laser light to thereby homogenize the light intensity. The photomask carries the image of an integrated circuit. The photomask is positioned between the illumination optics and a projection optical system. The projection optical system projects the image of the integrated circuit onto the semiconductor substrate. Both the illumination optical system, the photomask, and the projection optical system employ transmission optics. It was once thought that the limit of making integrated circuits using transmission optics would be somewhere around one micron; however, various improvements have been made such that one-tenth micron feature sizes are currently being produced.
One way of reducing the linewidth is to improve the quality of the optical components. Another way of reducing the linewidth is to reduce the wavelength of the laser light source. For example, KrF lasers, which operate at a wavelength of 248 nm, are capable of producing integrated circuits having linewidths approaching 120 nm. ArF lasers represent an improvement over KrF lasers, operating at a wavelength of 193 nm. With improvements to the transmission optics, integrated circuits can potentially be produced with linewidths as small as 70 nm. Designers are now considering F2 lasers. These lasers operate at a wavelength of 157 nm. F2 lasers hold the promise of producing integrated circuits having linewidths on the order of 50 nm.
While it may be possible to further reduce the operating wavelength of light sources used in illumination systems, the very use of transmission optics is becoming a limiting factor. The problem is that the glass materials currently employed in light transmission systems are not transparent at shorter wavelengths. Integrated circuit manufacturers have seen this problem coming for some time and are investigating ways of overcoming the above described limitations.
In one very promising approach, designers are considering reflective optical microlithographic systems that employ extreme ultraviolet (EUV) illumination sources. EUV systems operate at wavelengths in an approximate range between 11 nm and 13 nm. Instead of transmitting light through lens systems, reflective optical systems employ mirrors to direct the light onto the semiconductor substrate. The photomasks used in EUV systems are also reflective. Because the wavelengths in EUV systems are so short, any irregularity present on the surface of a mirror will significantly degrade system performance. Thus, the optical blanks used to produce EUV mirrors must be of the highest quality.
Currently, quality optical blanks can be produced using a flame hydrolysis process. A mixture of silica precursor and a very pure titania precursor are delivered in vapor form to a flame forming SiO2xe2x80x94TiO2 soot particles. The soot particles melt in layers into a solid fused silica optical blank. While this method can be used to produce high quality optical components for transmissive devices, this method has drawbacks when it is used to make EUV compatible optical components. One problem being encountered in the fabrication of mirrors is the presence of striae in the optical blank. The striae are formed as layers of soot are deposited during the flame hydrolysis process. Striae are less of an issue in photomasks than in optical components. Striae result in mid-frequency surface roughness. Another problem encountered is low frequency inhomogeneity which causes a phenomenon known as springback. Springback refers to the shape change of a glass object with a non-uniform coefficient of thermal expansion (CTE). The change in shape typically occurs upon removal of material from the glass object.
Another drawback relates to the presence of inclusions within the glass. Inclusions are either solid impurities or gaseous bubbles disposed within the glass. The occurrence of inclusions in glass made using the flame hydrolysis process is low, but improvements are always desired, especially for glass used in EUV applications.
What is needed is a method for producing EUV compatible optical blanks for use in reflective microlithography. A method is needed to produce homogeneous optical blanks having substantially no striae. Further, a method is needed to produce fully dense, and substantially inclusion free glass.
The present invention relates to a method for producing optical blanks for EUV microlithographic components. The present invention provides a method for producing homogeneous optical blanks having substantially no striae. The method of the present invention produces dense, substantially inclusion free glass. As a result, scattering is substantially reduced when EUV light is reflected from a component produced from the optical blank.
One aspect of the present invention, the present invention includes a method for forming an optical blank. The method includes providing a green body. The green body includes a non-porous exterior portion and a porous interior portion. The interior portion is evacuated to thereby create a vacuum in the interior portion. The green body is pressed using a hot isostatic pressing technique, whereby the green body is densified into a solid glass optical blank.
In another aspect of the present invention, the present invention includes a method for forming an optical blank. The method includes providing a green body having a non-porous exterior portion. The green body is a vitreous container having a hollow interior enclosed by a porous interior wall, the hollow interior being characterized by a volume capacity. The vitreous container is filled with a glass powder. A volume of the glass powder filling the vitreous container is substantially equal to the volume capacity of the vitreous container. The interior portion is evacuated to thereby create a vacuum in the hollow interior. The vitreous container is heated to render the vitreous container plastic. The temperature of the glass powder is raised to an appropriate compacting temperature. An external pressure is applied to the vitreous container. The external pressure collapses the vitreous container about the glass powder disposed within the vitreous container. The glass powder is fully densified to thereby form a solid glass optical blank. Subsequently, the densified solid glass optical blank is cooled.
In another aspect of the present invention, the present invention includes a method for forming an optical blank. The method includes providing a green body. The green body includes a non-porous exterior portion and a porous interior portion, the interior portion being a porous solid. The interior portion is evacuated to create a vacuum in the interior portion. The green body is heated to render the green body plastic. The temperature of the porous interior portion is raised to an appropriate compacting temperature. An external pressure is applied to the green body. The external pressure collapses the green body until the interior portion is fully densified, whereby a solid glass optical blank is formed. Subsequently, the densified solid glass optical blank is cooled.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.